An example of prior art is disclosed in Japanese Patent Application Laid-open No. Sho 55-65248, which proposes that a modified aromatic petroleum resin obtained by copolymerizing a specific thermal cracked oil fraction having a low indene content and a high vinyltoluene content with a phenol compound and turpentine oil be blended in a hot melt adhesive composition.
This modified aromatic petroleum resin is a preferred petroleum resin for use in hot melt adhesive, with its excellent compatibility with base resins such as ethylene-vinyl acetate copolymer (hereinafter referred to as "EVA"). However, practical adhesive strength in a temperature range above a certain point is not sufficient, partly because commercially available turpentine oil with a low diene value is used, as described in the embodiment of the foregoing laid-open patent application.
During the packaging procedure of liquid products, for example, where they are filled in vessels such as cans or bottles immediately after production while hot, then packaged in containers such as corrugated fiberboard boxes, heat may transfer from the products to the containers. Hot melt adhesive, which gains adhesive strength by solidifying, including the one described in the foregoing laid-open patent application, may easily lose the adhesive strength when exposed to high temperatures. Therefore, the containers need to be allowed to cool for a while until the products become cold enough. This cooling time inevitably prolongs the production time. It is for this reason that improvement in adhesion resistance to heat when applied to substrates, particularly paper substrates such as corrugated fiberboards, is awaited. Improvement in adhesion resistance to heat is generally achieved by increasing the molecular weight of the resin. However, increased molecular weight may sacrifice adhesive properties, especially low temperature adhesion.
An object of the present invention is to improve the adhesion resistance to heat and heat stability of the modified aromatic petroleum resin described in the foregoing laid-open patent application, while maintaining the good hot melt adhesive properties thereof, especially the low temperature adhesion.